Onioshidashi Park: Mount Asama’s Lava Field

In 1783 Mount Asama erupted with a violence that reshaped this corner of Gunma, sending out a lava flow that cooled into an ocean of jagged black rock. Locals decided demons inside the volcano must have pushed it out, and the name stuck: Onioshidashi. Today the lava field is a park, and walking its paths, with the volcano still steaming quietly on the horizon, is one of the more surreal experiences within easy reach of Karuizawa.

Walking the lava field

Paved walking loops thread between rock formations the size of houses, with longer and shorter courses depending on your energy. A temple hall stands among the rocks, raised in memory of those killed in the eruption, and it makes a striking sight with the black landscape piled around it. Look closely and you will find alpine plants colonising the crevices, at their best in the warmer months, while autumn brings colour to the surrounding hills and winter dusts the whole scene in snow.

Getting there

The usual approach is via Karuizawa, about an hour from Tokyo on the Hokuriku Shinkansen. From Karuizawa Station, buses heading toward the Onioshidashi and Kusatsu direction reach the park entrance in roughly 40 minutes; services are not frequent, so check return times before you set off. Drivers will find the park just off the scenic toll road across Asama’s northern slopes.

Frequently asked questions

How long does a visit take? Most people spend one to two hours, depending on which loop they walk.

Is it manageable with a pram or limited mobility? The main loop is paved and reasonably graded, though some side paths involve steps.

Is it open in winter? Yes, and the snow on black lava is genuinely striking, but dress for highland cold and check conditions.

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